1. The Field of the Invention
The present invention relates generally to biodegradable polymer blends. More particularly, the present invention relates to blends of two or more biopolymers, such as biodegradable polyesters and polyester amides, in order to yield sheets and films having improved physical properties such as flexibility and elongation. The biodegradable polymer blends may be suitable for a number of applications, such as in the manufacture of disposable wraps, bags and other packaging materials or as coating materials.
2. The Relevant Technology
As affluence grows, so does the ability to purchase and accumulate more things. Never before in the history of the world has their been such a large number of people with such tremendous buying power. The ability to purchase relatively inexpensive goods, such as books, tools, toys and food, is a luxury enjoyed by virtually all levels of society, even those considered to be at the poorer end of the spectrum. Because a large percentage of what is purchased must be prepackaged, there has been a tremendous increase in the amount of disposable packaging materials that are routinely discarded into the environment as solid waste. Thus, as society becomes more affluent, it generates more trash.
In many cases, packaging materials are intended for only a single use, such as boxes, cartons, pouches and wraps used to package many, if not most, commodities purchased from wholesale and retail outlets. Even the advent of computers and xe2x80x9cpaperlessxe2x80x9d transactions has not stemmed the rising tide of packaging wastes. Indeed, the onset of xe2x80x9ce-commercexe2x80x9d has spawned a great mail-order fad, thus increasing, instead of decreasing, the amount of packaging materials being used as products must now be individually packed in boxes suitable for shipping.
Moreover, the incredibly fast-paced lifestyles now being pursued have greatly disrupted traditional eating routines in which people prepared their own meals and sat down as a family or group. Instead, people grab food on the run, thus creating ever-increasing amounts of fast food packaging materials being used and then immediately discarded. In view of the rising tide of disposable packaging materials, some countries, particularly those in Europe, have begun to mandate either the recycling of fast food generated wastes or the use of packaging materials which are xe2x80x9cbiodegradablexe2x80x9d or xe2x80x9ccompostablexe2x80x9d. Environmental activists have also entered the fray to put pressure on companies that generate solid waste. Thus, large fast food chains such as McDonald""s have been essentially forced to discontinue nonbiodegradable packaging materials such as foamed polystyrene, either by government fiat or by pressure by environmental groups. There is therefore an ever-present need to develop biodegradable alternatives to nonbiodegradable paper, plastics and metals.
In response to the demand for biopolymers, a number of new biopolymers have been developed which have been shown to biodegrade when discarded into the environment. Some of the larger players in the biodegradable plastics market include such well-known chemical companies as DuPont, BASF, Cargill-Dow Polymers, Union Carbide, Bayer, Monsanto, Mitsui and Eastman Chemical. Each of these companies has developed one or more classes or types of biopolymers. For example, both BASF and Eastman Chemical have developed biopolymers known as xe2x80x9caliphatic-aromaticxe2x80x9d copolymers, sold under the trade names ECOFLEX and EASTAR BIO, respectively. Bayer has developed polyesteramides under the trade name BAK. Du Pont has developed BIOMAX, a modified polyethylene terephthalate (PET). Cargill-Dow has sold a variety of biopolymers based on polylactic acid (PLA). Monsanto developed, but has since stopped the manufacture of, a class of polymers known as polyhydroxyalkanoates (PHA), which include polyhydroxybutyrates (PHB), polyhydroxyvalerates (PHV), and polyhydroxybutyrate-hydroxyvalerate copolymers (PHBV). Union Carbide manufactures polycaprolactone (PCL) under the trade name TONE.
Each of the foregoing biopolymers has unique properties, benefits and weaknesses. For example, biopolymers such as BIOMAX, BAK, PHB and PLA tend to be strong but also quite rigid or even brittle. This makes them poor candidates when flexible sheets or films are desired, such as for use in making wraps, bags and other packaging materials requiring good bend and folding capability. In the case of BIOMAX, DuPont does not presently provide specifications or conditions suitable for blowing films therefrom, thus indicating that it may not be presently believed that films can be blown from BIOMAX.
On the other hand, biopolymers such as PCL, ECOFLEX and EASTAR BIO are many times more flexible compared to the more rigid biopolymers discussed immediately above. However, they have relatively low melting points such that they tend to be self adhering when newly processed and/or exposed to heat. While easily blown into films, such films are difficult to process on a mass scale since they will tend to self adhere when rolled onto spools, which is typically required for sale and transport to other locations and companies. To prevent self-adhesion (or xe2x80x9cblockingxe2x80x9d) of such films, it is typically necessary to incorporate silica or other fillers.
Another important criteria for sheets and films used in packaging is temperature stability. xe2x80x9cTemperature stabilityxe2x80x9d is the ability to maintain desired properties even when exposed to elevated or depressed temperatures, or a large range of temperatures, which may be encountered during shipping or storage. For example, many of the more flexible biopolymers tend to become soft and sticky if heated significantly above room temperature, thus compromising their ability to maintain their desired packaging properties. Other polymers can become rigid and brittle upon being cooled significantly below freezing (i.e., 0xc2x0 C.). Thus, a single homopolymer or copolymer may not by itself have sufficient stability within large temperature ranges.
In the case of the packaging of foods, such as refrigerated meats or fast foods, the packaging materials may be subjected to widely fluctuating temperatures, often being exposed to rapid changes in temperature. A biopolymer that may be perfectly suitable at room temperature, for example, may become completely unsuitable when used to wrap hot foods, particularly foods that emit significant quantities of hot water vapor or steam. In the case of meats, a wrapping that may be suitable when used at room temperature or below, such as at refrigeration or freezing temperatures, might become soft and sticky during microwave thawing of the meat. Of course, it would generally be unacceptable for a biopolymer to melt or adhere to the meat or fast food being served unless for some reason it was desired for the person to actually consume the biopolymer.
In view of the foregoing, it would be an advancement in the art to provide biodegradable polymers which could be readily formed into sheets and films that had strength and flexibility properties suitable for use as packaging materials.
In particular, it would be an advancement in the packaging art to provide improved biodegradable polymers which could be readily formed into sheets and films that were capable of being folded, sealed or otherwise manipulated in order to reliably enclose and seal a substrate therein.
It would be a further advancement in the art to provide improved biodegradable polymers which could be readily formed into sheets and films having sufficient flexibility while avoiding or minimizing problems such as undesired self-adhesion.
It would yet be an advancement in the art to provide improved biodegradable polymers which could be readily formed into sheets and films having increased temperature stability over a broad range of temperatures compared to existing biopolymers.
Such improved biopolymers are disclosed and claimed herein.
The present invention encompasses biodegradable polymer blends having improved strength, flexibility, elongation and temperature stability properties. Such polymer blends may be extruded, blown or otherwise formed into sheets and films for use in a wide variety of packaging materials, such as wraps, bags, pouches, and laminate coatings.
The invention achieves the foregoing improvements by blending at least one biopolymer having relatively high stiffness with at least one biopolymer having relatively high flexibility. For example, a blend containing a relatively stiff BIOMAX polymer, a modified PET sold by Du Pont, and the relatively soft or flexible ECOFLEX polymer, an aliphatic-aromatic copolymer sold by BASF, has been found to yield blends which have been shown to have strength and elongation properties which are superior to either biopolymer taken alone. Thus, the present invention has achieved a surprising synergistic effect of blending these two biopolymers.
BIOMAX is characterized as having a relatively high glass transition temperature and is highly crystalline at room temperature. Thus, BIOMAX tends to be quite stiff or brittle when formed into films or sheets. It also has poor elongation or elasticity. ECOFLEX, on the other hand, is characterized as having a relatively low glass transition temperature and is relatively amorphous or noncrystalline at room temperature, all of which contribute to the remarkable softness, elasticity and high elongation of ECOFLEX. Even so, the inventors have discovered the surprising result that various blends of BIOMAX and ECOFLEX actually exhibit higher elongation than ECOFLEX by itself, as well as higher break stress compared to either BIOMAX or ECOFLEX by themselves.
Other polymer blends have been considered, such as a blend of ECOFLEX, PLA and thermoplastically processable starch (TPS) and a blend of BAK and TPS. In each case, blending a biopolymer having a relatively low glass transition temperature with a biopolymer having a relatively high glass transition temperature has resulted in polymer blends that, in many cases, exhibit the desired characteristics of each polymer by itself, in some cases exhibiting even better properties, while diminishing or minimizing the negative properties of each biopolymer by itself.
In general, those biopolymers that may be characterized as being generally xe2x80x9cstiffxe2x80x9d or less flexible include those polymers which have a glass transition temperature greater than about 10xc2x0 C., while biopolymers that may be characterized as being generously xe2x80x9cflexiblexe2x80x9d include those polymers having a glass transition temperature of less than about 0xc2x0 C. The stiff biopolymers will preferably have a glass transition temperature greater than about 20xc2x0 C., more preferably greater than about 30xc2x0 C., and most preferably greater than above 40xc2x0 C. The flexible biopolymers will preferably have a glass transition temperature of less than about xe2x88x9210xc2x0 C., more preferably less than about xe2x88x9220xc2x0 C., and most preferably less than about xe2x88x9230xc2x0 C.
In addition, xe2x80x9cstiffxe2x80x9d polymers are generally more crystalline, while xe2x80x9cflexiblexe2x80x9d polymers are generally less crystalline and more amorphous. The relative stiff polymers, characterized as those polymers generally having a glass transition greater than about 10xc2x0 C., will preferably have a concentration in a range from about 20% to about 99% by weight of the biodegradable polymer blend, more preferably in a range from about 50% to about 98% by weight, and most preferably in a range from about 80% to about 95% by weight of the polymer blend.
The relative soft polymers, characterized as those polymers generally having a glass transition less than about 0xc2x0 C., will preferably have a concentration in a range from about 1% to about 80% by weight of the biodegradable polymer blend, more preferably in a range from about 2% to about 50% by weight, and most preferably in a range from about 5% to about 20% by weight of the polymer blend.
The biopolymers within the scope of the present invention are typically synthetic polyesters or polyester amides. Nevertheless, it is within the scope of the invention to also include a variety of natural polymers and their derivatives, such as polymers and derivatives derived from starch, cellulose, other polysaccharides and proteins. It is also within the scope of the present invention to incorporate inorganic fillers in order to decrease self-adhesion, lower the cost, and increase the modulus of elasticity (Young""s modulus) of the polymer blends. In addition, a wide variety of plasticizers may be used in order to impart desired softening and elongation properties.
In the case of sheets or films intended to be used as xe2x80x9cwrapsxe2x80x9d, such as wraps used to enclose meats, other perishable food items, and especially fast food items (e.g., sandwiches, burgers and dessert items), it may be desirable to provide sheets and films having good xe2x80x9cdead-foldxe2x80x9d properties so that once folded, wrapped or otherwise manipulated into a desired orientation, such wraps will tend to maintain their orientation so as to not spontaneously unfold or unwrap, as which occurs with a large number of plastic sheets and films (e.g., polyethylene). In order to improve the dead-fold properties of sheets or films produced therefrom, biopolymer blends (optionally including fillers) may be engineered so as to yield films having a relatively high Young""s modulus, preferably greater than about 100 MPa, more preferably greater than about 150 MPa, and most preferably greater than about 200 MPa. In general, increasing the concentration of the stiff biopolymer will tend to increase the Young""s modulus.
As discussed above, including an inorganic filler is another way to increase Young""s modulus. Thus, it has been found that adding significant quantities of an inorganic filler, such as greater than about 5% by weight, preferably greater than about 10% by weight, improves the dead-fold properties of sheets and films manufactured from such polymer blends.
Another way to increase the dead-fold properties is to increase the xe2x80x9cbulk hand feelxe2x80x9d of a sheet, which is done by disrupting the generally planar nature of the sheet or film. This can be done, for example, by embossing, crimping, quilting or otherwise texturing the sheet so as to have a series of hills and valleys rather than simply a planar sheet. This may be done, for example, by passing the sheet or film through a pair of knurled or other embossing- type rollers. Such texturing increases the ability of a sheet to take and maintain a fold, thus improving the dead-fold properties of the sheet.
Finally, another important advantage of utilizing biopolymers in the manufacture of wraps is that biopolymers are generally able to accept and retain print much more easily than conventional plastics or waxed papers. Many plastics and waxes are highly hydrophobic and must be surface oxidized in order to provide a chemically receptive surface to which ink can adhere. Biopolymers, on the other hand, typically include oxygen-containing moieties, such as ester or amide groups, to which inks can readily adhere.
In view of the foregoing, it is an object of the invention to provide biodegradable polymers which can be readily formed into sheets and films that have strength and flexibility properties suitable for use as packaging materials.
It is another object and feature of the invention to provide biodegradable polymers which can be readily formed into sheets and films that are capable of being folded, sealed or otherwise manipulated in order to reliably enclose and seal a substrate therein.
It is a further object of the invention to provide biodegradable polymers which can be readily formed into sheets and films having sufficient flexibility while avoiding or minimizing problems such as undesired self-adhesion.
It is yet an object of the invention to provide biodegradable polymers which can be readily formed into sheets and films having increased temperature stability over a broad range of temperatures compared to existing biopolymers.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.